The present invention relates to an exhaust system for a turbomachine, such as a steam or gas turbine or the like. More specifically, the present invention relates to an exhaust system for an axial flow turbomachine that minimizes the strength of harmful vortices within the flow.
The performance of a steam turbine may generally be improved by lowering the back pressure to which the last row of blades of the turbine is subjected. Consequently, turbines often discharge to a condenser in which a sub-atmospheric pressure is maintained. Typically, the exhaust steam discharging axially from the last row of blades is directed to a condenser mounted below the turbine by turning the flow 90.degree. from the axial to the vertically downward directions. This turning of the flow is accomplished by an exhaust system that includes a diffuser in flow communication with an exhaust housing.
Diffusers are generally comprised of inner and outer flow guides that serve to increase the static pressure by reducing the velocity head. Typically, the cross-sectional shape of the outer flow guide is a simple arcuate shape--see, for example, U.S. Pat. Nos. 3,945,760; 4,863,341; 3,058,720; 3,697,191; and 3,690,786. However, conical shaped diffusers have also been utilized--see, for example, U.S. Pat. No. 4,391,566. Although outer flow guides are generally of uniform axial length, at least one steam turbine manufacturer has utilized an outer flow guide in a bottom exhaust system that has an axial length that varies around its circumference, being a maximum at the bottom of the diffuser and a minimum at the top.
The exhaust housing receives steam from the diffuser and directs it to the condenser through a bottom outlet opening in the housing. To obtain maximum performance, it is important to configure the exhaust system so as to minimize losses arising from the formation of vortices in the steam flow. However, as explained below, the difficulty of this task is exacerbated by the somewhat torturous path the steam must take as it is directed to the condenser.
The steam from the diffuser enters the exhaust housing in a 360.degree. arc. However, it discharges from the exhaust housing to the condenser through only the bottom outlet opening. This presents no problem with respect to the steam flowing in the bottom portion of the diffuser since by turning such steam into the radial direction, the diffuser turns the steam directly toward the bottom outlet opening. However, the steam discharging at the top of the diffuser must turn 180.degree. from the vertically upward direction to the vertically downward direction, in addition to turning 90.degree. from the axial direction to the vertically upward direction. Consequently, vortices are formed within the exhaust housing in the vicinity of the top of the diffuser outlet that create losses in the steam flow that detract from the efficiency of the exhaust system and, therefore, the performance of the turbine.
One approach for minimizing such losses used in the past involves the incorporation of flow dividers into the exhaust diffuser that allow the steam to expand and turn into the radial direction through several smaller concentric flow passages, rather than a single large flow passage, as disclosed in U.S. Pat. No. 3,149,470 (Herzog). Another approach, suggested for a gas turbine exhaust system, involves the use of flow stabilizing ribs formed on the outer diameter of the diffuser that guide the flow toward the outlet opening so as to prevent the formation of vortices, as disclosed in U.S. Pat. No. 4,391,566 (Takamura). However, such approaches have not been entirely successful and can result in a considerable increase in the manufacturing cost of the diffuser.
It is therefore desirable to provide an exhaust system for a turbomachine capable of turning an axial flow discharging from the turbine into a radial direction, such as vertically downward, in such a way that the formation of vortices and other loss mechanisms are minimized. It is also desirable that the shape of the exhaust diffuser in such an exhaust system facilitate its manufacture, thereby minimizing the cost of the diffuser.